Common designs for high power inductors and chokes used in switching mode applications have constructions incorporating coils with bobbins for carrying the windings and isolating them from a permeable core. The core, which is typically manufactured with assemblies comprising U-shaped core-parts, thereby acts as a magnetic flux path. The two U-shaped core-parts then are arranged adjacent to each other, abutting on front side contact surfaces of their legs. One or more bobbins are accommodated on one or on two oppositely situated legs of the core-parts. Inductive elements of such a design are commonly manufactured by sliding prefabricated coils onto the legs of the core-parts. The coils thereby comprise a bobbin carrying the winding of the coil usually having a supporting and an isolating function. The core-parts are usually combined in a way that the contact surfaces abut on each other inside the coil bobbins thus forming air gaps inside the bobbins.
Such designs have several disadvantages. The winding window introduced by the bobbin results in a less than ideal copper fill factor. The winding windows of bobbins are particularly disadvantageous since the bobbins have to be manufactured with comparatively large tolerances in order to avoid difficulties during assembly of the inductive element, i.e. inserting the legs of the core-parts into openings of the bobbin. Besides the bad copper fill factors the large tolerances also may be responsible for vibrations of the coils. Another significant disadvantage of the use of a bobbin is a high thermal resistance between core and winding due to the thermal insulating properties of the bobbin material.
The U.S. Pat. No. 4,326,182 describes a U-core transformer having bobbins molded directly on the U-shaped core-parts outside the region where the core-parts contact each other. While improving the above mentioned standard transformer design, the U.S. Pat. No. 4,326,182 still uses bobbins for carrying the winding. Moreover, the method of manufacture for such a transformer is complex and involves the step of molding a bobbin onto the core and handling the U-shaped core-parts, which are non-symmetric with respect to the winding axis, when performing the winding process.